Recent advances in the development of semiconductor and organic light-emitting diodes (LEDs and OLEDs) have made these solid-state devices suitable for use in general illumination applications, including architectural, entertainment, and roadway lighting, for example. As such, these devices are becoming increasingly competitive with light sources such as incandescent, fluorescent, and high-intensity discharge lamps.
An advantage of LEDs is that their turn-on and turn-off times are typically less than 100 nanoseconds. The average luminous intensity of an LED can therefore be controlled using a fixed constant-current power supply together with pulse width modulation (PWM), for example, of the LED drive current, wherein the time-averaged luminous intensity is linearly proportional to the PWM duty cycle. This technique of using PWM signals is disclosed in U.S. Pat. No. 4,090,189. Today, PWM is typically the preferred method for LED luminous intensity control in that it offers linear control over a range of three decades (1000:1) or more without suffering power losses through current-limiting resistors, uneven luminous intensities in LED arrays, and noticeable colour shifts as identified by Zukauskas, A., M. S. Schur, and R. Caska, 2002, Introduction to Solid-State Lighting. New York, N.Y.: Wiley-Interscience, p. 136. The PWM signals used to control the LEDs are preferably generated by microcontrollers and associated peripheral hardware.
According to U.S. Pat. No. 4,090,189, a plurality of LEDs can be connected in parallel with their anodes connected to a common voltage supply, and their cathodes each connected to a different fixed resistor and switch. The fixed resistors can serve to limit the peak current through each LED when the corresponding switches are closed. Practically however, this only works well if the forward voltage of each LED is nearly identical, otherwise different values of resistors must be chosen for each different LED to prevent current hogging by any one LED in this parallel configuration. This use of resistors can also induce large losses thus reducing the overall efficiency of the circuit.
Alternately, as in U.S. Pat. No. 6,621,235, a technique of using transistor current mirrors for each parallel string of LEDs is described as a way to equalize the current shared by each string. Another technique is disclosed in U.S. Pat. No. 5,598,068, which sets up multiple independent current sources for each parallel string of LEDs. These techniques however, typically use a large number of components and have a low efficiency.
Another means to address forward voltage differences in parallel strings is through forward voltage binning which is not necessarily practical in terms of the additional step during the production process. This procedure can additionally result in wasted parts.
In addition, with the invention of high brightness light-emitting diodes (HBLEDs) and the desire to use many of them in luminaires for architectural or general illumination results in LED circuits with a plurality of parallel strings, each containing a plurality of LEDs. Due to manufacturing tolerances, as well as fundamental differences between the device chemistries of LEDs of different colours, the forward voltage of different LEDs can vary by up to approximately 1.6 volts. This disparity in forward voltage requirements can be compounded when several of these LEDs are stacked in series, with the result being that parallel strings of the same number of LEDs can have large forward voltage drops. Driving LEDs using the above cited techniques means that the common voltage source must be of a high enough voltage to bias the LED string with the largest forward voltage drop. As a result, the LED strings with a lower forward voltage requirement will have excess voltage, which will result in excess power dissipated by the components in series with the LEDs that are used to limit the current across the LED string with the lower forward voltage drop. If this form of dissipation was not provided, excess current will flow through the LED string with the lower forward voltage drop which can overdrive the LED string and result in LED damage.
An advantage of PWM techniques is that the average LED current can be efficiently controlled by reducing the duty cycle of the PWM switching signal to prevent exceeding the maximum rated average current. In practice however, this means that if LEDs, or strings of LEDs, with different forward voltages are in parallel with each other, all drawing power from a single voltage source, the highest forward voltage string can be fully dimmed from 0 to 100%, whereas the lower forward voltage string must be driven with a maximum duty cycle, Dmax, of less than 100% to prevent overdriving. FIG. 1 shows a lighting system configuration in which a microcontroller or similar device 13 is used to generate PWM signals for each LED string 11 to 12, each drawing power from voltage source 10. This configuration has two problems. First, assuming the PWM signal generator 13 has 8 bit accuracy, for example, which can provide 256 discrete dimming levels for 0 to 100%, then for the strings with Dmax<100%, the dimming resolution would be significantly reduced. For example, if the maximum ‘safe’ duty cycle was 75% for a particular LED string, then the number of discrete dimming levels for that string would be reduced to 75%×256=192. Secondly, firmware can become more complicated since different LED strings must be driven with different duty cycles to achieve the same level of effective dimming, thereby resulting in a requirement for custom calibration factors to be determined for each LED string for storage in EEPROM (electrically erasable programmable read-only memory), for example. These problems would also typically apply to any other digital control method known in the art that could be used to vary LED brightness, for example, Pulse Code Modulation (PCM).
Therefore, there is a need for a low cost and efficient method and apparatus for scaling the current provided to LEDs and other light-emitting elements that allows each type of light-emitting element to be dimmed from 0% to 100%, without the need for complicated firmware.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.